text:
How do "fundamentally unstable" aircraft deal with the risk of computer failure?
As technology becomes more advance airplanes are becoming less and less stable.  In flight school I learned that it's dangerous to load a Cessna out of it's CG range, and now in college (for aerospace engineering) I'm learning about modern fighters and their fundamentally unstable designs.
A lot of these unstable aircraft are supermaneuverable but rely on computers to fly them more than the human pilot in the cockpit.
I'm sure that these airplanes are thoroughly checked before every flight to make sure the computer systems will perform accordingly, but in the event of a failure what kind of side affects might a pilot experience?
The two conditions I can think of are

No computer aid, but the control surfaces are still "responsive"
The pilot loses all control of the aircraft and none of the inputs work.

Military aircraft don't care about safety. They have ejection seats.

@JanHudec not true. Survivability is important as pilots (and aircraft) are expensive and slow to replace, repairing them is much prefered.

Actually, the X-29's (shown above) wiki page has an excellent paragraph discussing the 6 (!) computers used to control the plane via FBW.

In short, they do not. Many modern aircraft (usually top-end fighters) would shred themselves within fractions of a second without constant, active corrective (computer) control. Even the Stabile ones would die, due to uncontrolled turbine behaviour, incorrect computer-applied trim, etc.

"fundamentally unstable" but über-responsive aircraft have multiple computers that take a vote on everything the pilot wants to do. If one computer fails the others will take over. There is often no direct connection between the control column and the flight surfaces so the "no computer aid" situation would be the same as #2.
If everything fails, the checklist is rather short: eject immediately.

Unfortunately you seem to be conflating a two distinct issues in one question.

Computer control of aircraft.
Unstable aircraft design.

First dealing with computer control, or flight control systems. Many modern designs military and civil are using digital fly by wire (FBW) systems. This has little or nothing to do with aircraft stability. These systems often have no mechanical backup, so in reality the pilot is merely instructing the computer how to fly the plane. Modern FBW systems use a quadraplex system, i.e. there are 4 duplicate computers. 3 are active and one is hibernating. Then if one system goes duff the other 2 can switch the duff one off and bring the 4th out of hibernation. If you only had 2 computers how do you know which one is wrong? - hence 3 are used. Prior to flight clearance these systems are tested with thousands of hours of rig testing, safety checks etc. Whole volumes have been written on the subject.
Second issue - unstable aircraft design. Again a couple of comments, first aircraft instability levels are not driven by manoeuvrability requirements - modern military aircraft are aerodynamically unstable for supersonic performance not manoeuvrability. Modern FBW systems allow designers to explore unstable designs but there is a limit. Originally the FBW systems were looking at pitch instability but now it is being extended to lateral instability for some of the more novel stealth designs.
So in terms of your two proposed conditions - if there is a mechanical backup then pilots can adapt and are capable of controlling slightly unstable configurations in the event of a failure, although they may whinge a little about the handling qualities. For a fully digital FBW system then for your proposed conditions then as the systems failed there would be a gradual degradation in flying qualities but there would still be a backup system allowing a return to base capability. To have a total loss in control then there would have to occurred a series of multiple failures. Again risk and safety assessments are another whole volume of work.

"aircraft instability has nothing to do with manoeuvrability" Really??

Yes - aerodynamic instability is used to reduce the trim induced drag supersonically. As you go supersonic the the aerodynamic centre moves aft. If you start from a stable design subsonically this requires large angles to trim in the supersonic region and a performance penalty. If you are unstable subsonically then in the supersonic region you don't need the same flap angle to trim hence better performance. There is an argument that an unstable design is detrimental to manoeuvrability. All the pilot wants is to point the nose and unstable design is harder to stop in the right place.

I was just questioning your general statement that aircraft instability has nothing to do with maneuverability, which is not true. In general, the more maneuverable an aircraft is, the less stable it is. By definition

Sorry but we seem to be talking about subtly different aspects hence the differences. I've edited the response to try and be clearer about what I meant in the context of the question. The original question isn't clear and specific so it's difficult to get the level of detail correct. Whilst I agree that on a conventional aircraft there is the link between manoeuvrability and stability once you stick in a full authority FBW system you can effectively tailor the system to your requirements. It's entirely possible to make an unstable aircraft unresponsive, or less manoeuvrable.

Using FBW you can make an naturally unstable design fly like a very stable aircraft, but the opposite is not true. You can modify a Typhoon to act like a Cessna Citation, the Citation is never going to roll and pitch like a Typhoon. +1 For decoupling FBW from fundamental stability though. You can fly unstable aircraft without FBW and stable aircraft are often produced with FBW.

Most modern fighter aircraft are not mechanically linked.  A massive failure in all computer systems (probably as a result of battle damage) would leave the pilot no choice but to eject.

@Adrian: Or you can move the aircraft's center of mass to a different location at supersonic speeds (for instance, by moving fuel around, like the Concorde did), which lets you avoid having a large trim-drag penalty at supersonic speeds but still have an inherently stable aircraft at subsonic speeds.

In the event of a computer failure it boils down to two factors,

Manual control - For a naturally unstable aircraft, manually controlling it depends on the pilots skill, which I am sure they are trined for. The safest method would be to manually take control and emergency land.
Eject - If th aircraft is completely uncontrollable despite pilots efforts, the pilot has to eject.

This is not a bad answer, and in fact would be the decision process the pilot must make.  "Unstable" planes can be made more stable by: 1. reducing speed, to where it is more stable (transsonic to subsonic).  2. adding drag to the tail with speedbrakes, flaps, rear gear, or even a parachute.

It also, effectively, repeats the 2 points made in the original question, therefore doesn't really provide an answer.

@Robert, yes, this happens, but rather as an exception. Watch how a test pilot lands an unstable MiG-AT with a failed control system and damaged tail.


text:
Do similar angles on a plane make it more stealthy?
I learned in my intro to engineering class at my college that 5th gen fighters (which are stealth aircraft) are designed with similar angles to evade radar.
Why is this true?
Also, does the angle matter?  Many of these modern fighters looks to have very similar angles (no pun intended).

Short answer: Yes.
Long answer: There are two ways to prevent the radar energy from being reflected back to its source in order to avoid detection:

Absorption, and
Directed reflection.

Absorbers work well only over a small wavelength band, so some reflection is unavoidable. Now the idea is to collect all reflected energy along specific directions, so the aircraft will reflect very little energy in all other directions.
Radars can adjust their sensitivity, and if they operate at their most sensitive setting, they will detect many false positives, but also a stealth aircraft. If the stealth aircraft maneuvers, it will momentarily shine one lobe of the reflected radar energy back at the radar receiver, causing it to reduce sensitivity such that the weak returns in all other directions will not be registered. Compare this to a diamond which will reflect light back at specific angles, causing it to sparkle.
There are two-lobe designs (Northrop YF-23 and B-2) and four-lobe designs (Lockheed F-117 and F-22), but the idea is the same in all cases. Reflected radar energy is focused in narrow beams, so very little is given off in all other directions.
This is not to say that such aircraft can evade detection. When the radar stations are networked, they can pretty well trace those sparkles and stitch together the aircraft's location. Also, NATO aircrews which had participated in international maneuvers were less than impressed with the stealth capabilities of their colleagues, but were under a gagging order to be more specific.
And then there is infrared. What does it help to be invisible on radar if the aerodynamic heating of the leading edge can be picked up by an IR receiver from 300 km away? When B-2s were flown over to the Farnborough airshow, British Eurofighters could detect them far out over the Atlantic already.

Does "their colleagues" mean Russia? The US? Someone else?

@raptortech97: The Russians are much less sanguine about stealth, and those maneuvers were in the western states of the US. So the NATO aircrews were western European, and the colleagues were US pilots.

The angle matters because radar works kind of like a mirror. Imagine holding a mirror out in front of you. If you start to twist it in any direction, you reach a certain point where the light coming off of you is no longer bounced back into your eyes, but is reflected off in another direction. Radar however cannot ever be completely reflected away, but this same principle allows a significant portion of the radar's energy to be deflected off in a useless direction, instead of back toward the dish.
The angles are similar because that angle was determined to have the smallest radar signature. So it makes sense that every other facet on the same (geometric) plane would be at the same angle. This means that the radar's energy is directed away in a very specific direction (away from the source), rather than a bunch of different angles (that would be more likely to return the energy to the source).

Good answer. I would add the following into analogy: imagining shinig a powerful flashligt on a disco ball and a flat mirror 100 meters away. You would not see the reflection on the mirror unless it's almost exactly pointed at you; disco ball, on the other hand, would give a clear "back signal" from any angle. That's why F-117 was all flat surfaces.

@DarkWanderer Thank you, now I can't put the idea of F-117 shaped disco, ehm, ball out of my head.

I assume you're using the term 'similar angles' in a geometric sense, to mean that the angle from a reference like the aircraft centerline to the lines made by two structural features are the same.  Your photo shows this well, with the wing leading edge angle the same as the horizontal stabilizer leading edge angle and the same as the inlet (?) angle.
I believe the reason for this is that the overall likelihood of radar detection is lower because each of these features will reflect radar signal in the same direction as one another.  This means that the reflection will be very much stronger in the direction they reflect, but that reflection has a very narrow spread.  In contrast, the features will not reflect radar strongly at all other angles.  Thus the radar dish has to be "lucky" to be at exactly the right angle from the aircraft to receive the strong signature, and so the overall probability of detection is reduced since at most angles the reflection signature is reduced.  This is also the reason that flat surfaces are used in many stealth designs.

I think the OP is talking about the similar orientation of the leading edges of body, wings and horizontal stabs. What i am missing in all of the answers is why the leading edges would be important at all. I can understand that the large area parts acting as reflectors (wing undersides etc) would be oriented in particular ways. But the edges seem only having a tiny cross-section in comparison.

Define "with similar angles". In itself the phrase means nothing.
The basic shape of the airframe is of course a function primarilly of its required function to provide lift and enough inner volume for the aircraft's components.
Stealth depends on several factors:

absorption
scatter
deflection

These are functions of the shape and materials used. Creating surfaces that cause incoming EM waves to scatter in many directions rather than being reflected directly back causes the return signal received by a RADAR system to be much weaker, reducing the detection range.
But a big part is also played by placing grids in air intakes, shielding sharp edges with sawtooth overlays (see the canopy and bay doors, for example), and covering different parts of the aircraft in different materials.

The phrasing is fine. He's saying similar angles as in all of the new stealth aircraft look very similar to each other. Similar planform, similar fuselages. Not only that, but on each individual aircraft, the angle of the wing sweep is similar to the angle of the elevator sweep is similar to the vertical stabilizer sweep, and so on.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What are the requirements for logging SIC time?
I am going from KPRC to KVGT with a friend in his C-182.  I don't have a high-performance endorsement, but can I -- as a private pilot -- log as SIC at any time during our flight?
If so, what rules do I have to follow?
From what I understand:

I can log SIC time since the aircraft is a SEL
I can't log landings, obviously
And SIC time really doesn't mean anything

Am I right in assuming those things?

"log SIC time" is the phraseology used by the FAA, not "log as SIC". See the FAA document I referenced in the checked answer.

You can basically log SIC time if you are a required crewmember and are not able to log PIC time.
So, generally in an airplane like a C-182 which is certified for only one pilot, you can't log SIC time because you aren't a required crewmember.
There are a few exceptions though, like being a safety pilot for the PIC while they are using a view limiting device (like an IFR hood), etc.
SIC time is not pointless because it still adds to your total time!
As far as landings, you can log them only if you are the sole manipulator of the flight controls, in which case you are allowed to log PIC time (even though you aren't allowed to act as the PIC since you don't have your high performance endorsement).  Oddly enough, in this situation the pilot who is acting as PIC isn't allowed to log it because they don't meet any of the requirements for doing so. It's one of the only situations where the PIC isn't allowed to log it as such.

Let us continue this discussion in chat.

So if you are a student, and a required crewmember, do you log both "student" and "SIC" time?

@ssummer Depends on the student.  If they are a student pilot (do not have their pilot license yet) then they are never a required crewmember unless they are solo (in which case they log PIC time).  Once they get their license then it depends on whether or not they are PIC or not (but yes, if they are not the PIC and are a required crewmember then they would log SIC time and Dual Received."

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What are the US regulations governing experimental aircraft?
There are more experimental class aircraft in the USA than any other.  That being said, what type of requirements does an experimental aircraft have to meet?
Also, are there ways around certain regulations?  For example, the guy in the picture below was battling with the FAA so they would let him fly his jet pack in the USA over the grand canyon, which they originally said no to, but they eventually let him - is this because he is "famous" and therefore has less rules he needs to follow?

or the guy adjusted for the regulations for low pass over a national park

Your first question

There are more experimental class aircraft in the USA than any other. That being said, what type of requirements does an experimental aircraft have to meet.

is very broad and should probably be its own question.
As to your second question

Also, are there ways around certain regulations?

The short answer is "Yes".  You can apply to the FAA for a waiver of specific regulations.  It's commonly done for aerobatic contests to permit aerobatics below 1500', within 4 nm of an airway, etc.  The waiver is of extremely limited duration and calls out the regulations to be waived.  Air shows also have them.
I haven't personally requested such a waiver but I've seen them.
The FAA probably permitted the overflight because the pilot was able to show it was safe, would do no harm, and wouldn't cause them any problems.


text:
What prep work can I do before transitioning from SEL to SES?
I'm very interested in getting my SES rating.  Is there any personal studying or prep work that I can do to minimize time in the air and with an instructor?
EDIT: I do have my SEL pilot license already.

Read this?

If you already have a pilot license for single-engine land airplanes you've done a huge amount of prep work already - you know all the airspace, navigation, airwork, etc. that you need to know for your seaplane rating.
Brush up on all of that in the seaplane as you can be tested on it and the aerodynamics will be a bit different with floats hanging out there, but you're already well ahead of the game.
You need to learn how to take off, land, and taxi/sail all over again on water (which is arguably the hard part) but the bulk of your existing knowledge will transfer over.
For studying and prep I imagine the FAA Seaplane Operations Handbook (FAA-H-8083-23) would be the best place to look - it's pretty comprehensive.
Also if you can commandeer a seaplane pilot for some hangar flying (or better a pilot and aircraft for a few real-world touch-and-gos & some practice sailing the plane on water) that would obviously be a great advantage, but coming to your first lesson familiar with the Seaplane Operations Handbook would probably put you well ahead of the curve anyway.

I've familiarized myself a bit with that book already, but I'll make sure that I know it really well before I start my first lesson.  Also, I like the sound of "commandeering" an airplane hahaha.


text:
During VTOL landings, do Harrier pilots cut power before touching the ground?

When vertically landing, do Harrier pilots cut power before they touch the ground?

Well, they obviously have to at least reduce power, otherwise they could bounce back up, due to backblast from the ground. The same power which makes you lose altitude at 10 meters might make you hover or even gain altitude at 0.5 meters.

On the carrier I imagine the deck is always rocking up and down in the 1-2 meter range. So it's pretty dangerous to stay at that height for too long, might as well slam it. It's more or less like landing a helicopter on a ship I guess.

Cutting the power just before touchdown in VTOL aircraft appears to be appropriate in certain circumstances. After watching 20-30 landings
  on Utube most where soft landings. However two landings on pitching
  ship decks appear to show the airplane's decent increase within a few
  feet of the ground consistent with the power being cut while in the
  air. 2:15 and 0:25
Not mentioned in the information below is
  the reduction of FOD and the tendency for the Harrier to spall and
  break up concrete in just a few seconds. - J. Zumwalt

[ From the Harrier manual - information omitted before the quoted paragraph given by YMB1 ]
Yes, Harrier pilots are known for ‘slamming’ their jets on the ground by cutting power in the last few feet. In fact, for a Harrier, a smooth landing is a scary landing.
The reason pilots cut the engines a few feet from the ground are primarily two:
1) To kill the high potential energy stored in the craft while it is hovering and prevent it from turning into kinetic energy which may render the craft uncontrollable. When the harrier is hovering in the air, the engines are working at a high power setting. The complete weight of the aircraft is thus balanced by the engines alone, since the lift component from the airfoil (wings) while hovering is zero.
2) To prevent the extremely hot jet blast from bouncing off the ground and getting sucked into the engine again, thereby damaging the engine.
-Sagar Dhawan

They do cut power a few feet above deck, especially if the carrier is in rough water.
If the VTOL (F-35 included) carried full power landing onto the deck, the LG will take on some weight, and instantly the plane is overpowered and tends to rise up. The engine cannot spool down quick enough in that instance to prevent this “inadvertent takeoff”.
This condition is worsened in bad weather, as the deck is also moving up and down. So if the plane has full power on the way down and the deck is moving up, it will be thrust up into the air, possibly out of control.
So, it is best to cut power a few feet up so the plane sticks the landing and stays safely on deck while the engine goes to idle. And for a moving deck, pilots train for it, all a matter of timing.
The LG is designed for such “heavy landings”. - Joel Berson[

Harriers will drift side to side if the pilots sit hovering close to the ground for too long. It has Caused accidents in the past. So once the pilot is confident in his approach, they reduce power to mitigate the length of time in the zone where they may drift sideways because of the thrust rebound. - Megan Portero

Your discounting the 100-200ms sound travels in 100-200ft to the photographer AND turbine inertia that lags several seconds behind a change in throttle. There is also no way of knowing how close the audio is synced to the video. Both aircraft do drop the last few feet.

"Sometimes" can be a valid answer. It does not have to be "always" or "never" :)

All three references are to Quora, not exactly the most credible referencing. If you’re going to accept your own answer I think you need a more credible source.

Just to add a bit to the difference of opinion in some of the answers above, an excerpt from The Jet Engine issue 5 by Rolls Royce, page 85 about STOVL aircraft with thrust vectoring nozzles:

Particular performance issues raised by this include the potential for hot exhaust gases to enter the intake - potentially leading to surge or stall - and the need for the engine to decelerate rapidly on landing. This requirement for a rapid deceleration is because, on a constant velocity vertical descent, engine thrust equals aircraft weight, and so when the aircraft lands it will tend to bounce and then rise at a similar rate.

And the punchline:

Thrust has to be quickly reduced to prevent this, leading to potential surge of the low-pressure compressor.

They don’t.  If you watch carefully they set power for a gentle descent around 1-2 ft/s then pull power to idle on impact with the deck.  Cutting power in a vertical landing prior to touchdown either results in a really firm touchdown and possibly damage to the landing gear or airframe, depending on how high the pilot is prior to touchdown.
Landing a Harrier in s conventional manner on a runway is possible.  The aircraft handling much like any other high performance fighter in the roundout.

Actually, they do. I found an answer by a Harrier Pilot on the Quorum forum. The correct answer is very interesting. In the pilots explanation, a "soft" landing is considered a "bad" landing and gives a very good explanation.

@jwzumwalt, have a link to that particular question/answer? Seems relevant to include that in this discussion :).

Well a 1-2 ft/s is hardly a soft landing.  I agree that they want to plant it, but they are not cutting power until they make contact with the deck.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What are the scale effects of GA wing tip airfoils?
[ update: I am adopting an improved description provided by Peter Kämpf: "...only looking at the airfoils in isolation..." ]
The goal of this question is to determine if wing tip airfoil scale plays a significant part in stall characteristics for aileron control.
Lets assume we have a small Piper or Cessna aircraft with only a change in size from root to a tapered wing tip. There is no washin or washout at the tip. Lets further assume the wing tip has 50% chord to the root (if some other value is useful, feel free to use what is appropriate). If Reynolds number has any appreciable bearing on the result, lets use approx 50kt/mph as a ruff stall speed.
Q1) Will the wing tip stall at the same speed as the root (root vs tip), or is there an appreciable difference do to size & scale?
Q2) What would the stall characteristics be (root vs tip) if we scaled the tip chord 50% but did not change the thickness?
Illustration for reference, values are not exact.

May I recommend airfoil tools on the web.  Moderate taper with some washout might be workable.  "Endcapping" the wing may also help reduce tip stall to keep it safe.  Airflow, lift, and stall characteristics of the design should be studied.

T2 Buckeye perhaps?

If you only look at the airfoils in isolation, the shorter tip airfoil will stall at a slightly lower angle of attack and lift coefficient (I use angle of attack instead of speed, as both should be equivalent). It has only half the Reynolds number of the root airfoil and, therefore, viscosity forces are twice as big relative to inertial forces. This is made worse by the ailerons on the outer wing: A downward deflection will strain the boundary layer more, moving stall to even lower angles of attack.
The exact difference depends on the airfoil. Below I pasted Figure 18 from chapter 4 of Sighard Hoerner's book Fluid Dynamic Drag. Your wing has a chord of 6 ft at the root and 3 ft at the tip, corresponding to a Reynolds number of 3.25 Million at the root and half as much at the tip at 50 kts speed. Note the logarithmic scale of the X axis below: The 2 million point is halfway between the 1 million and the 10 million point.

But there is more: With a 50% tapered and untwisted wing the highest loading is at maybe 60% of span - that is where the full wing will stall first. Next, you have to assume a fuselage at the root. Depending on the local pressure distribution, the root might even begin to stall earlier. You sure know comparisons like the one below for the lift distribution and stall behavior of different planforms (source). Yours is case C (moderate taper):

My recommendation is clearly against the proposed planform. Combined with aileron loads, it will be very prone to tip stalls. Given the "right" airfoil, it will show positive roll damping, that is, it will roll all by itself once the stall starts on one side. There are several steps to make the wing more benign:

Use a thinner airfoil (10%-12%) at the tip than at the root (here 15%-17% are a good choice). Why that helps is shown in Figure 23 below, again taken from chapter 4 of Sighard Hoerner's book Fluid Dynamic Drag.
Use less taper. 70% tip chord relative to root chord gives the best value for induced drag and leaves more margin against tip stalls.
Use an airfoil with benign stall characteristics. Definitely avoid 5-digit NACA airfoils.
Use at least 2° of washout. That sounds little, but has a profound impact on stall behavior.

Q2) What would the stall characteristics be (root vs tip) if we scaled the tip chord 50% but did not change the thickness?

You mean the same absolute thickness, so the tip airfoil has twice the relative thickness than the root airfoil? This would guarantee very poor stall behavior and is clearly a poor layout. While the root will benefit from a thicker airfoil (lighter spar, more internal space for landing gear and fuel), the tip enjoys no such benefit. Fuel near the tips is more often a liability (roll inertia, tank internal pressure in spins). Manufacturing a spar with a single taper is only marginally more complicated than a spar of constant thickness. The flanges need to be tapered in both cases - this creates the most work.
In order to have the highest margin for aileron deflection, the recommended tip thickness is 10% - 12%, not more. Many designs even used a 9% tip airfoil successfully. Also, the use of differential aileron deflection is highly recommended. Let the trailing-edge-down movement of the aileron be only half as much than the trailing-edge-up movement. This will improve stall resistance and help to reduce aileron forces, especially at high angle of attack.
My favorite example for a poor wing layout is the Dornier Seastar. It was designed more than 40 years ago by a bunch of old Dornier engineers who basically repeated what they had done on the Dornier 17 another 40 years earlier. While the first prototype inherited a rectangular wing from the Do-28, the "new" wing design used a NACA 23015, no washout and had a taper ratio of 0.7. During an early test flight, the airplane flipped on its back during low speed trials. Fortunately, this happened with enough distance to the ground so this upset could quickly be corrected, but it necessitated the addition of a drooped nose on the outer wing.
Now look at the Beech Bonanza: Same planform and very similar root airfoil (NACA23016.5), but a NACA 23012 at the tip and 3° of washout. And much better low speed characteristics. Usage of a NACA 6-digit airfoil would improve those further, see the Piper Cherokee for example.

Kampf  the stall points seem to correspond with the area of highest lift, which logically implies different areas of the wing have different Clift at same AOA.  Some type of airflow phenomena. The elliptical is symmetrical, tapered somewhere around C to be close to matching it.  Is there an optimal LE/TE sweep ratio for tapered wings?

@Peter Kämpf - This more than adequately answers Q1. But for Q2, I thought the blunter the nose section the higher the stall angle? I didn't know if the roundness is the absolute physical size or it's proportion - hence Q2? From what I can tell, Cessna appears to use the same airfoil scaled down (+washout), but Beech and Navions (nearly the same airfoil) go from a thick fat root to a thin under camber at the tip. I am exploring the idea that a thicker tip = more fuel, w/o to much penalty. And, constant spar dimension is easier to mfg.

@jwzumwalt: Made the answer more clear regarding Q2.

The trapezoid wing is a shape we are seeing in our modern fighters, but has been around in various forms and different proportions and shapes since the 1930s, when designers become aware of the effects of drag on higher speed range. Early designs featured a straight front and a tapered back, later designs swept both front and back, the supersonic F16 has a swept front and straight back, the F35 has swept front and back.  Roll rate is also improved with this design, along with the ability to handle higher G loads.
(Reverse) tapering the back seems to focus on drag reduction, the front on improving high speed performance.  But the old Hershey, hailing from the 1920s and earlier, clearly is a very good choice for GA aircraft.
It's stall characteristics, starting at the root and providing plenty of buffet, add a degree of safety that is very important in this class of aircraft.
Although thickening the wing tip would help (this has been applied to deltas), it is basicly an attempt to fix what has been undone with the taper.
Hershey wings are very easy to manufacture, basicly rolling a rectangular sheet right over the ribs and spar.  A trapezoid with varying thickness could be done with greater difficulty.
And the trapezoid can be improved with downwash, slats, and flaps, albeit with greater cost and complexity.
Depending on the application, it is a viable alternative.  For simple GA, Hersheys rule.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is this aircraft that has both piston and jet engines?
A friend sent me this picture of a trimotor aircraft with what looks like two radial engines and a jet in the middle. He didn't know what is was and neither do I. Anyone recognize it?

It is a double-hulled "YAK-110" (which is two Yak-55's), it was only recently completed and will be at Oskosh 2018 (and so will I, so I may be able to add additional pictures in August).
Here is an article about it.

Here is another video with the jet engine at its first airshow (and the picture you have is a screenshot from it):

It also isn't the only aircraft set up this way (jet/piston), the Screamin Sasquatch is another airshow aircraft converted to have both.

I have control!  No!.. I have control!     You had control yesterday!... This is left cockpit control day!  No!... It's right cockpit control day!

@JohnK Sounds like a reasonable question.

Of course it was.  I was making a joke about the configuration.

@JohnK - "The left cockpit is for immediate loading and unloading of pilots only. There is no stopping in the right cockpit." "No, the right cockpit is for loading of pilots and there is no stopping in a LEFT cockpit!". "The LEFT cockpit has ALWAYS been for loading and unloading of pilots. There's never stopping in the RIGHT cockpit!". "Don't you tell me which cockpit is for loading, and which cockpit is for stopping!"

At least on the Twin Mustang the right side cockpit was the radar guy so there wasn't any question who was flying.

Not only the Screamin' Sasquatch (what a name!) but how about the B-36 Peacemaker?

authors: Bob Jarvis - Слава Україні


text:
Why is the front section of the Su-30 not aligned with the longitudinal axis of the body?
Please take a look at the picture of the Su-30 below. You will notice that the front section of the Su-30, from the nose to the canopy, is not in alignment with the longitudinal axis of the body. The canopy is raised vertically above the extended axis, and the front portion points downward.

In US fighter jets like the F-18 the canopy is not raised vertically above the extended axis and the front portion aligns with the axis from nose to tail.

So why is the front portion of the Su-30 shaped in that manner? Is it to give the Su-30 any advantage over aircraft that do not have such a front portion?

Both noses are curved.  What do you mean by "front portion?"  What do you mean by "curved?"

@mins Could you please help us out, then, by editing the question to make it more clear?  Many of us just aren't understanding the question.  Even the person who answered admits to having to guess.

@WayneConrad: "Could you please help us out, then". Well, my pleasure. The front portion has been defined as "right from the canopy up to the front radar", which is not a surprise. And it is visible that contrary to many aircraft this section axis seems pointing down, not forward. So overall the centerline of the full fuselage is not a straight line but a curve.

Oh, I see: There's been an edit that clarified it.  I apologize that I missed it.

I could draw a similar straight line on the SU-30, and a curved line on the Hornet.  I kinda get what you are asking, but like many designed objects, they are simply shaped differently.  There isn't a dramatic enough difference, or enough clarity in the question to make this a useful question in my opinion.

This is not necessarily the best way to look at this airframe;  it is better observed in a 3-view to understand this.

Viewed from the side and top, the main fuselage does not curve down but blends into pancake like lifting body which mates with the wings and empennage of the aircraft; the engines then are slung under the fuselage in a linear pylon which blends into the fuselage at the tail the design is beneficial for several reasons.

Fuel capacity: The Flanker family of aircraft all have very long legs and can carry a lot of gas due to this flattened mid section and makes a good solution for the need to carry a lot of fuel.
Aerodynamically clean: the blended wing/body design offers low drag as well as low wing loading, making the aircraft very maneuverable as a consequence of high drag during high alpha flight.
Stores carriage: blended wing/body offer a lot of ordnance hard points (12 to be exact) for carriage of weapons, fuel tanks, sensors, etc.

The forward action of the fuselage is based upon the the SU-27UB airframe with an aft crew station raised in order to better facilitate flight instruction in the airplane this required raising this section of the fuselage.  The nose and radome appear to droop down but are laid out that way to increase the real estate for a large radar antenna in the nose radome for increased detection ranges.
Like most East Block war machines, the SU-30 is brutally functional, refined only where needed and passed up for utility elsewhere.  While it may appear to 'curve' the fuselage is essentially straight and the engines aligned with the COG.

"Like most East Block war machines, the SU-30 is brutally function" - Isn't that true for any war machine?

It would be helpful to also show a picture of a F-18 to show the difference. This would improve the quality of this (already good) answer

"While it may appear to 'curve' the fuselage is essentially straight" It looks pretty curved to me. Look at the side view: the rear of the canopy is the highest point of the fuselage and the whole thing is essentially /-shaped.

We are students of aerospace engineering at the University of Stuttgart. We don’t know if we understood you fully, but it seems that you mean the Su 30 nose, which is pointed downwards a little bit.
A typical design consideration of this type of aircraft is the field of view for the pilot.
The F 18 is used for aircraft carrier operations, but at high angle of attack approaches the long nose reduces the forward visibility. With a shorter and downwards pointed nose, we don’t have that problem.
We can compare the Su 30 to the Concorde, which also has a downwards pointed nose for landing purposes. The problem of reduced visibility was so severe that the engineers had to design a mechanism to adjust the nose. Furthermore, the shape of the nose had a major influence on the strength of sonic booms. Probably the nose design of the Su 30 is somehow optimized to mitigate sonic booms.
The design of an aircraft’s nose is highly influenced by the radar. That could have also led to the resulting shape.

How does this affect aerodynamics?

Its for energy-maneuverability reasons. Like everything else in engineering its a trade off. 1st important fact to understand is this: In airliners, and most previous gen combat aircraft, designs are (primarily) made for flying in level (ie cruise) most effectively. At this optimal cruise point/line, the best engine fuel efficiency and aerodynamic efficiency, the lift/drag ratio, are closely matched. This is the reason why airliners have much higher wingloadings. They are great at cruise, but has to solve the slow speed performance with overly complicated high-lift devices like 3 stage flaps..
4th gen aircraft are designed for preserving energy while turning, so they DO NOT actually cruise at near their best L/D ratio.
Points below are the mathematical part to explain the sentence above, feel free to skip to the next paragraph.. When calculated with data from MiG-29's aerodynamic manual;
-A MiG-29 cruising with full fuel and weapons payload (14900kg) at 6000m (20k feet) altitude at M0,85 requires lift coefficient of Cl=0,145 to stay in the air; that corresponds to overall L/D of 6,5.
-Best L/D MiG-29 is capable of which is 10,5@Cl=0,4.. That lift coefficient equates to MiG-29 pulling 2,76Gs at same flight conditions..
-Even when Cl=0,85 which equates to pulling 5,86Gs, the MiG-29 is more aerodynamically efficient than cruising at same flight conditions.
-If payload or altitude get smaller, necessary Cl will decrease, or in other words, for the same Cl and L/D plane will pull even higher Gs. On extreme example, MiG-29 with 50% fuel at sea level is more aerodynamically efficient while pulling 9Gs at M0,85 than flying straight in level. (This is not to say it requires less thrust overall; it means it requires less thrust per lift generated.)
While all calculations about Cl may seem irrelevant to nose angle, it is not.. Cl is also directly proportional to angle of attack (AOA). Summary from all these calculations is this: irrelevant of weight or flight conditions, if you fly MiG-29 even at 12 deg AOA manuvering, it will be more efficient than flying it straight at just 1 deg AOA.. Point where Mig-29 is most efficient is maneuvering at 4 deg AOA, not cruising at 1 deg. This is a physical necessity for energy efficient maneuvering and applies to planes like F-16 as well.. An F-4E can turn well, but cannot really sustain that turn. These different design choices and optimizations is what "designed for sustaining high G turns" actually mean.
Getting back to Su-27/30.. I think the basic reasoning is quite obvious at this point. Nose of the aircraft doesn't create any meaningful lift, it creates only drag, reducing the L/D, and making the plane less efficient. If you make the nose sit at a certain angle like ~6 degrees in Su-27, it makes aircraft more efficient at around that angle of attack, but its less efficient at the others.. In the most basic sense, by pointing the nose downward, designers trade level flight fuel efficiency and acceleration performance at the higher speed + lower altitude parts of the flight envelope.. At the same time they get better sustained turn performance and better acceleration on the low speed high altitude part of the envelope. MiG-29, F-15 and F-16 also has their nose pointed slightly downwards by the way. Not as extreme as Su-27 so they are only visible in 3-view drawings but still..
From more advanced perspective, it gets even more complicated.. Su-27 uses airfoil sections throughout its body; its the only true blended wing + lifting body design ever created (Though MiG-29 looks very similar it isn't aerodynamically, its more comperable to F-16 than Su-27). A cross section of Su-27 from right next to its nacelles is still an airfoil. A cross section from its nose to the tail stinger is still an airfoil if you exclude the cockpit canopy. On a such complex design no one could say for certain how flow behaves for certain.. Su-27 is a true marvel of aerodynamics engineering, and since newer designs has to take other considerations like stealth shaping and have a fat fuselage with internal bay, it may very well be the peak of what aerodynamics engineering will ever create; as far as maneuverability is concerned that is.

check the L/D polars for +6 degrees.  In other words, the nose is 0 AOA (in flight), while the wing is +6 AOA.  The B-52 bomber does the same thing.  Long range for the SU-30 was a great clue.

"4th gen aircraft are designed for preserving energy while turning, so they DO NOT actually cruise at near their best L/D ratio." -- I don't understand the significance of this answer to the original question.  As explained in this related answer https://aviation.stackexchange.com/questions/77467/what-is-the-main-source-of-yaw-stability/77478#77478 , an aircraft must bend like a banana (convex side down) to stay streamlined to the curving relative wind in a steady-state turn.  The Su-27 fuselage appears to be curved in the opposite direction.

Just a guess: the SU-30 (unlike the F-18) is a two-seat cockpit. It derives from a "combat trainer", where the rear seat too presumably needs to see out the front of the aircraft (so the rear seat is much higher than the one in front -- to see over the front one's head.)

It's so the guy in the back seat can see out the window without looking into the back of the head of the guy in the front seat.

A hypothesis could be drived from how the Su-27 and MiG-29 were developed. The  Central Aerohydrodynamic Institute provided both projects with optimal wing studies. The wings were then "dirtied" with details such as cockpit, engines, fuselage for avionics and fuel, and empennages. The plane is basically divided along the wing plane - cockpit and fuslage above, nacelles below - with lifting-body characteristics (similarly found on the F-14).


text:
What is this sound for when starting the engine of an F-15?
If I remember correctly, to start up old fighters such as F-4s, you need an ASU but it looks new fighters do not need an ASU. Today, I was watching this video. It's a so cool video to know how new fighters start their engines. From 2:38, something starts and make a sound just like woooooooooooooooooo... you think a wolf is howling. From 3:06, the sound becomes stronger but, the question is what kind of starter is used by P&W F100 that makes such a sound?

Well, the F-4 normally used an Auxiliary Power Unit, APU (or ASU as you call it), but it also had the capability to use *Start Cartridges" which were pyrotechnic cartridges the size/shape of a gallon can that were inserted into slots in the auxiliary air doors, and when fired, produced high pressure gas to drive a turbine that rotated the engine fast enough to get it started.. But they were prone to misfire and so were used infrequently.

@CharlesBretana That sounds like an answer!

Except that it doesn't speak to how F-15s and F-16s do it, which,  I think, (but don't know for sure), is because they have an internal  APU.

Aha! finally found the answer. Based on this and this F-15 uses an APU named Jet Fuel Starter (JFS) which is fitted centrally between the two engines. See 15 on the figure below

Another figure of the intake and exhaust of the JFS from bottom of the fighter:

The below figure shows the danger area of the JFS:

And finally here is an image of this JFS:

Hope this helps.


text:
Can someone identify this warplane silhouette?
I saw this picture today in a news article about the current conflict in Iraq/Syria.

The caption claims it is a US warplane, but I don't recognise the silhouette. Can someone identify this plane?

That there?  Oh, that's 240 tons of supersonic, nuclear capable FREEDOM.

This is a B-1B Lancer.
It is a 4 engine (afterburning engines mind you, very rare for a bomber) variable sweep wing bomber, designed during the Cold War to use its terrain-following radar to stay low and fast, weaving between the mountains of Russia to stay below radar to deliver nuclear warheads.
It carries more bombs than the B-52 as well.
Essentially, it is a bomber built like a fighter. It is actually the only swing-wing aircraft in the entire US military right now.
Fun fact: The canards near the nose aren't for control. Because the aircraft is flown by autopilot using terrain-following radar (TFR), weaving into and out of mountains, autopilot makes many quick pitch adjustments. The canards are to cancel out sine waves in the structure of the fuselage.
It can carry a whole lotta' bombs:
24 nuclear bombs, 48 JDAMs, 84 Mk82 bombs, or 48 AGM missiles.
100 were built, 66 remain.
I leave with my current desktop background:

24 nuclear bombs. That is pretty crazy powerful.

@Peter: Only the small 1.2MT ones ;)

@Peter penetration missions, it lobs small ones at airfields, radar sites, SAM batteries, for the B-52s with their megaton range weapons to have a clear path to their targets. Used to be they carried some 20 SRAM missiles but those were retired in a cost cutting attempt so now it's gravity bombs, putting the crews in risk to save the beancounters some $$$ signs.

AFAIK the B-1B is the only supersonic US military B-series aircraft in operation, although the B-2 is faster at <1000' MSL.

@HCBPshenanigans had to steal that sweet-looking background :-)

@zamnuts You are correct.

@raffian Consider it a donation!

Mother of God @ payload capacity O.O

@PlasmaHH 1.2MT may be small, but it's still way more than I'd ever want to be around when they go off.

@Jules: in case 24 of them do, I don't know if thats not maybe the better option...

@jwenting: Wouldn't the B-1 be the U.S. bomber best-suited to surviving the blast of its payload, given that it's the fastest of the three and carries fairly low-power nukes?

@Sean Honestly? When it comes to nuclear payloads, they don't really care if the aircrew survives. Cold fact, but true. Once they make it to the target, that's good enough for SAC.

@ChrisM.: That would be the AFGSC; the SAC was disbanded in 1992.

@Sean this was the mission profile created for the B-52 when the B-1A was canceled. The B-1A would indeed have been a fast penetration bomber thought to not need that operational profile. The B-1B was envisioned as a more survivable replacement for the B-52 and would use the same profile, but blast itself a narrower corridor.

@ChrisM. yes and no. Over time crew survivability became ever more important as nuclear war was becoming something that was meant to be limited in scope, so bombers returning to bases for reloading with either nuclear or conventional weapons for a second wave of strikes became important.

The F-14 was also swing-wing so either there are two of those aircraft types active or I missed it when the remaining F-14 were deactivated.

They've been retired for over 10 years now, haha.

Yep. That's a B-1B Lancer: B-1 on the Wikipedia
I'm surprised to see it over Iraq, I thought they were using smaller attack aircraft, F-15E's for example. The B-1 is a substantial (and, I can report, extremely loud) bomber with a big load.
They US have been flying them since the mid-80's.

You aren't lying about loud. Spent the night in a tent on MacDill AFB once and couldn't hear for an hour.

it seems to be rather high up. Loud doesn't matter there... And a single B-1 needs both less people in danger, and is less loud as well, as say half a dozen Strike Eagles. Loudest I've ever experienced was a full squadron of F-4s taking off in formation, 3 abreast, on full burner.

Hey, why use a mallet when you can use a sledgehammer?

@Sean precision strike is ever more important. It both limits collateral damage, allows you to strike closer to your own or friendly forces, and is cheaper. In case of the use of heavy bombers at the moment, the heavy bombers employ precision weapons, their size allowing them more time over target and the ability to strike more targets in a single mission. Plus their range means the dependency on forward operating bases in potentially unreliable countries is greatly reduced.


text:
What is the name of the maximum altitude at which a helicopter can fly?
What is the name of the maximum altitude at which a helicopter can fly?
I recall it as "vertical limit", but I can't find a Wikipedia page for it other than a movie of the same title, which seems to concern mountain climbing altitudes.

Like with fixed wing aircraft, it is called the ceiling. Usually two ceilings are distinguished;

service ceiling, where the aircraft can still achieve a positive rate of climb of either 100 fpm (propeller) or 500 fpm (jet).
absolute ceiling, the theoretical altitude at which no positive rate of climb can be achieved. In other words: the maximum altitude at which the aircraft theoretically can fly under maximum engine power.

So does that imply there's a sort of "certified ceiling," the maximum altitude the helicopter is certified to be safe (i.e. not have a retreating blade stall?)

Helicopters also have a stat you might have seen called In Ground Effect (IGE) and Out of Ground Effect (OGE) Hover altitudes.  This is the maximum altitude (MSL) the helicopter can hover at when it is in or out of ground effect.  Essentially, when IGE, a helicopter can hover at higher Mean Sea Level (MSL) altitudes because they get a little help from the proximity of the ground, which reduces the rotor tip vortices (reduces the drag on the rotor). This site has some good info on ground effect as it relates to helicopters.

Also consider that "hover" means no forward airspeed. If you are trying to land on top of a mountain it's unlikely you will have calm air, you will have 20-50kt forward airspeed against zero groundspeed.

'hover' means 'no forward ground speed.' The ASI is nearly always active in a hover.

Under 14 CFR Part 29.1527, the Maximum operating altitude for a transport-category helicopter is:

§ 29.1527 Maximum operating altitude.
  The maximum altitude up to which operation is allowed, as limited by
  flight, structural, powerplant, functional, or equipment
  characteristics, must be established.

So its not just a matter of power or thrust, or even just retreating blade stall, but rather the entire design envelope for the helicopter must be taken into consideration for maximum altitude.
For example, the Robinson R-66 has a number of operating limitations that pertain to factors other than maximum thrust:

And there is a Vne chart which limits forward speed to prevent retreating blade stall at high density altitude:


text:
What are the problems Ukrainians have with F-16?
Ukraine say they will not be able to operate F-16 till the end of the year, even if they get them. The communication does not explain, why.
It is anything known on what are exactly they problems? Is it difficult to master, or is the maintenance not ready? I cannot image that runways intended for the passenger airliners are not good enough for this interceptor.

Also, see this:
How long does it take, in average, to train for a different plane?

If to use F-16 to hunt drones, the airports are where they should be - quite close to the large cities the interceptors would be defending. HESA Shahed 136 is unarmed and flies 185 km/h. Looks like a good target to start from.

Training of pilots and ground crew are the main cause of the delays according to this article from the Washington Post.
Relevant parts from the article (emphasis mine):

KYIV, Ukraine — A first group of six Ukrainian pilots is not expected to complete training on the U.S.-made F-16 before next summer, senior Ukrainian government and military officials said, following a series of delays in the West’s instruction program for the sophisticated fighter jet. ....

But after the start of training was pushed back several times, Ukraine will now probably have to endure another year without the fighters, which officials in Kyiv hope will provide a significant military edge amid a slow-going counteroffensive and help better protect civilians against Russia’s regular missile and drone strikes.

U.S. officials describe a training effort, rather than hobbled by Western foot-dragging as Ukrainian officials charge, that is only slowly getting off the ground because of the complexities of standing up a new international initiative and the challenges Ukraine must face in identifying aviators with the right skills in the midst of an existential fight.

Despite the United States’ own far larger training capacity, the Biden administration, in a shift from many of the other initiatives surrounding Western support for Ukraine, appears set on letting European allies lead in the F-16 effort for now at least.

Though the pilots are already fluent in English, the officials said, they must first attend four months of English lessons in Britain to learn terminology associated with the jets. That instruction will occur along with ground staff who may be less proficient in English because, according to Ukrainian officials, Denmark requested to train entire crews together rather than just the pilots first. Denmark’s ministry of defense declined to comment.

The F-16 airframe:

is able to pull 9 g's;
is a fly-by-wire aircraft;
and is controlled by a (largely) fixed side stick.

Transition from any other fighter to the handling qualities of this plane does take a long time, particularly given the high speeds and manoeuvring actions involved - in addition to any aviation English courses, learning to fly this particular aeroplane is tricky and takes time
From this answer:

The F-16 can do 9G in manoeuvres, and the pilot is inclined backwards with their arm on an armrest. If this arm needs to move a stick it shifts position, and during a manoeuvre can get grabbed by the load factor - precise control is much easier if the hand, wrist and arm can stay in one position.

@RodrigodeAzevedo yep. It has a centre stick and an 8g limit


text:
Why was the F-14 retired 15 years ago while the contemporary F-15 is still operational?
The F-14 and the F-15 were designed in almost the same years and each one was at the top of its own "compartment" (respectively, US Navy and US Air Force).
However, almost 15 years ago, the F-14 was retired in favor of the F/A-18, surely less powerful but probably more multi-role (while the F-14 was improved too for ground attack), while the F-15 was not superseded by the new F-22.
So, the F-14 was replaced by a project that already had 16 years on its shoulder in 2006 — the F/A-18 was a project of the 80's — but F-15 continues to be side-by-side with the F-22.
Why these complete different approaches to fleet renewal?

There aren't enough F-22s to go around. Too expensive.
The F-18C/D Hornet that replaced the F-14 has itself been mostly replaced by the F-18E/F Super Hornet. The look the same, but the E/F is actually larger and more advanced. Not really the same aircraft.
Maintenance. Facilities on a carrier are restricted, simply due to space. If they can reduce the number of airframes to support on a ship, and still accomplish the mission, good.
Lack of mission for the AIM-54, which was the primary reason for the F-14.

The chief reason was to save money.  At the turn of the 21st Century, the Navy saw shifting to the new F/A-18E/F Super Hornet for all seaborne tactical strike operation (and eventually electronic warfare with the new EA-18 Growler aircraft) offered the Navy a fleetwise annual savings north of $1 billion over a multi-airframe tactical aircraft fleet.
There have been several proposals to update the existing F-14 airframes and even purchase a new state of the art variant of the F-14 called the Super Tomcat 21, which Grumman had proposed as the base of a new family of strike fighter aircraft based upon the F-14D. The Super Tomcat 21 would have included a host of new features such as

F110-GE-429 turbofan engines. This 29K pound thrust class engine, a modification of the same variant found in later block F-16s and now some Strike Eagle derivatives, albeit with a longer exhaust pipe tailored to the Tomcat, would give the ST21 supercruise performance (>Mach 1 without the use of gas-guzzling afterburner) while carrying a relevant air-to-air loadout. Some state M1.2-1.4 supercruise would have been possible.

Enlarged and recontoured leading-edge gloves with an additional 2,200lbs of fuel storage each.

Underfuselage-mounted Ford Aerospace (now Lockheed Martin) Night Owl targeting pod/FLIR and navigation pod.

Single-piece windscreen for enhanced visibility.

Wide-angle raster-scan HUD capable of projecting FLIR imagery.

Glass cockpit.

New mission computers and graphics processors.

On-Board Oxygen Generations System (OBOGS)

AN/APG-71 radar with additional upgrades ported over from the AN/APG-70 used on the F-15E. Expanded range and capabilities over the original AN/APG-71 in F-14D, which itself was an outgrowth of the AWG-9.

Digital Flight Control System (DFCS).

Wet wing pylons capable of carrying 300-gallon external tanks.

Ability to carry 425-gallon external tanks on nacelle hardpoints if it was developed. Existing tank size 280 gallons.

Larger multi-segmented fowler-flaps.

Enlarged and extended slats on wing leading-edge.

Bring-back to ship capacity increased from 9k lbs to 16k lbs.

Reduced approach speed and better slow-speed control.

Integration of latest standoff weaponry, as well as AIM-120 AMRAAM.

Upgraded AN/ALE-47 countermeasures dispenser.

Integration of the BOL countermeasures dispensers into the outboard pylons as a mission configuration option.

Such an aircraft would have provided the Navy with the capabilities of, more or less, a navalized F-15E Strike Eagle. But even with all the advanced upgrades that the Super Tomcat 21 would’ve offered, it was still seen as obsolete in the advent of stealth technology, which the Navy really wanted to get available in fighters on its carrier decks.
It is possible that the Navy may have bought Super Tomcat 21s or similar A/C utilizing Low Observable Technologies as a replacement for existing F-14s had the planned successor to the A-6E Intruder, the McDonnell Douglas/General Dynamics A-12 ‘Flying Dorito’ not been such an failure.  Facing massive cost and weight overruns and the Navy unable to justify it, the A-12 program was terminated in 1991 by then Defense Secretary Richard Cheney.  The proposed NATF, or Navalized Advanced Tactical Fighter variant of the F-22 airplane, then in development in the ATF fly off, was abandoned by 1994 - a victim of post-Cold War defense cuts.
Faced with dwindling defense budgets in the Clinton administration, the Navy was looking for a more conservative and cost-effective option, which it found in a modernized variant of the F/A-18 airplane.  This also offered the advantage that the Navy could package and develop the jet under the existing F-18 program, eliminating the need to ask Congress for additional funds for a new and risky fighter program.  Finally with Joint Advanced Strike Technology (JAST) program, which eventually morphed into the Joint Strike Fighter / F-35 family, the new modernized F/A-18 airplane offered the navy a more fiscally sound interim solution for fleet defense needs until JSF and other stealth aircraft were operating from their carrier decks.
Today in the face of continued developmental problems with the F-35C, the Navy continues to upgrade the F-18 Super Hornet with the Advanced Super Hornet program.
As for the F 15, it, too, sojourns on simply because the Air Force does not have an available option currently as a replacement. Both the F-22 and F-35 aircraft programs are problematic and costly, greatly reducing the number of aircraft that were purchased.  In the case of the F-22, the need for 750 Raptors to counter a Soviet threat which no longer existed.  A  limited asymmetric war on terror in the 2000s also made the F-22 something of a dinosaur despite being the most sophisticated fighter aircraft on the planet; an airplane with no war left to fight.  Perpetual  development troubles and setbacks with F-35, reducing annual procurements and limiting fleet size.  This leaves the Air Force with no other choice currently but to upgrade it’s existing F-15, F-16, and A-10 fleet of aircraft until a more suitable replacement becomes available.  It would be a sad irony indeed, if F-22 squadrons began to be replaced by F-15EX units!
UPDATE:  Dear God, my predictions are coming true!
https://www.thedrive.com/the-war-zone/44954/air-force-wants-to-retire-33-f-22s-buy-more-f-15exs-in-new-budget

Good discussion. As someone who spent the majority of their AF career in acquisition, I can say the answer is always about the money. Missions change. Solutions change. The entire process is driven by the budget cycle. It's a never-ending cycle under the obscurely named Future Years Defense Plan (FYDP). It used to be the Five Year Defense Plan until they went to a two year cycle stretching it to 6 years. Labeled it "Future Years" instead of "Six Year" so they could keep the same acronym. The only thing that's assured is that you won't get what you ask for.

There are two main parts to answer this question:
First part is a list of reasons as in the other answer:

F-14 was too optimized for one reason, BARCAP (Barrier Combat Air Patrol: when an interceptor rushes from a point to a far-away target to stop its advance), that seemed to be less needed in the 1990s. And it was difficult to adapt its airframe (the F-15 was more adaptable because it had more room, while the F-14 is tiny)
F-18 was a good replacement for all missions, while the F-16 or the F-22 had problems in replacing F-15: F-22 was too expensive, F-16 was already stretched in multiple allied sales (and was anyway not an equivalent of the F-15).

Second reason is that, sometimes, high-level decision-makers take a decision that appeared later to be a blunder. This case could be made, and has been made, about the retirement of F-14. Because F-18 lacks range and speed to do efficient BARCAPs, that seems to be needed again against re-emerging threats (cruise missiles, bombers). F-18 is not adapted to fight against CHinese fighters above Taiwan while their carrier will have to escape balistic missiles. The point here is that reasons that motivated the retirement of F-14 in the past could be challenged with today's point of view.

"the F-14 is tiny" -??

Yes the F-15 had more room and was more adaptable than the F-14, which needed heavy modification of its center (move of the motors...) in order to place new things (especially new electronic défensive devices). Of course, including the wings, F-14is not tiny

"multiple allied seelings" - care to clarify that one? Looks like a typo, but I'm not sure for what.

I suppose "sellings" should have been "sales", but I still don't understand the point. The F-16 is one of the "teen fighters". It wouldn't replace the F15, it was a cheaper single-engine alternative. And for that reason, the F16 too is still sold.

Can you please define BARCAP?

@MSalters You're right and that's actually what I'm saying: the F-16 was not a replacement for F-15. Extensive optimisation of military finance could have sent the F-16 in the US Air Force, but it would have been undoable


text:
What would the reason be for this helicopter maneuver?
At 4:30 in this YouTube video, a Chinook traverses the runway on its rear wheels. The text on the video lists "Emergency landing and take-off". Is there any reason for the pilot to do this other than "because I can and it's an airshow"?

The pilot then proceeds to back down the runway on the rear wheels, a maneuver that I can only imagine is showing off, but again, is there a legitimate flying need to do this?

Some BV-347's (modified from the Chinook) were fitted with a retractable rear-facing gondola which allowed the pilot to control the helicopter from underneath facing rearward. That being said I can't see a need for this type of ground maneuver other than showing off... Maybe to reduce ground speed quickly and maintain directional control? That's all I can think of.

I am not a rotocraft pilot, but I don't think helos normally "taxi" the way aircraft do. They just land and takeoff from the same spot. The maneuver shown would be relevant if for some reason you needed to take off from some other place where you are, or needed to maneuver the aircraft on the ground in some weird emergency. If you needed to get a helicopter into a shelter or building immediately (like somebody is shooting at you) and there is no time for a tow you might try something like this.

@TylerDurden Many flights and helicopters taxi, either on the ground when wheeled or in a hover taxi.

Take a look at this picture of a rooftop deployment: https://379e5bfa-a-62cb3a1a-s-sites.googlegroups.com/site/operationhooah/Home/CHINOOK_ROOF_LANDING_IN_Afghanistan.jpg?attachauth=ANoY7cr4LLyMosRhPvOmHKw6QPJ3iUN6s4D6C733TksX68LJtCdhf9gEV_NgvewDqelsyEmumh0VuDUOwHbOYSEvBEMmE-zuoaGpfIY92wgMkzvC8YvLZWu024Z0_UqEdeQafULrAJEAUPwbxwFCL9VWkba8prtryoyj6DE154YpIxuDT0FAvrdmO8_LhN7qizwfVa33pq_C0-JfCQ6Poo1Z-K4WQ9t6g8HcsLE42XxZhpspZfRA9JQKLHYv0lDPWQWPt_xzV5T4&attredirects=0 via http://www.operationhooah.com/Home/cool-chinook-stuff

That is not a runway.  That is a cliff by the looks of things.  And that would be an excellent way of offloading troops quickly without loitering for a full length landing spot, which would open yourself up as a target longer than necessary. (based on the screencap image of the video)  But yes, a lot of the airshow footage is showing off lol.

Ah, that's true, @NZKshatriya, but if you read the question, I am  not talking about the cool picture that was selected for the "cover" of the video, instead I reference 4:30 in the video (which is where the clip starts when you hit play).

Ex-Royal Air Force helicopter force here, spent a lot of time around Chinooks.
I think the main answer is "because I can", but I can think of a legitimate combat use.
You are heavy, and forced to land downwind into a narrow strip because there is a hazard on the upwind side of the landing zone.  There is not enough room to turn around.  You could hover taxi backwards like this to pick up some inflow into the rotors from the relative airflow (and therefore increased lift), then climb away backwards until you do have enough room to transition into normal forward flight, more or less as shown in the video.
This is where team work kicks in - you've really got to trust your load master who is telling you exactly where you are.
Never seen it done though.

Very interesting, thanks, Simon! That would account for the "back taxi", any legitimate reason for the forward taxi like that, beyond, "because I can"?

@FreeMan Sorry, I can't think of one.  The first half is a "quick stop", the wheels on the ground help a little bit to scrub off the speed but the second half, I don't know.

It looks like it's done to showcase the impressive level of skill & control that the pilot is able to achieve w/ this aircraft. Sort of like the Ken Block Gymkhana video series, where he's showing off amazing car control, but you would never want to drive like that except in a drifting contest.
Stretching the definition of 'practical', if you needed to tow something a short way & you had no pickup truck available, but somehow you did have a Chinook helicopter w/ a world-class pilot.... this would kind of be an option
Source: I just enjoyed the mental image of someone using a Chinook as a pickup truck

The first paragraph largely repeats the existing answer and the second is, as you say, a stretch. In fact, a huge stretch. I'm pretty sure that anything you could tow in this way could be moved easier and more safely by just getting a bunch of guys to push it.

You're right. I guess we could always come up semi-plausible use cases for anything, but given that my example is something that will probably never happen, do you think I should just delete this answer?

Deletion is up to you.

@NathanSmith No, you should not. +3-0

When our CH- 46 (PHROG) pilots are moving the aircraft on the runway they were directed by ground crew like myself, there are plenty of times we would taxi them forward and backward so I don't believe it would be "just because" or "showing off". It's part of every pilots training as is the rear wheel landing or "pinnacle landing" which is used to land on rooftops and mountain tops hence pinnacle.

Thanks for the input! They would taxi with only one pair of wheels on the ground, instead of all 4? What was the reasoning behind that? I could see it being practice for a pinnacle landing, but both in images (in this Q/A) the helicopter appears to be nearly horizontal (it's hard to be precise from the angles of the pictures) so the "pinnacle" portion seems to be just a regular landing on a ½ size landing pad - it doesn't seem to be in the "wheelie" position.

@FreeMan: With all four wheels on the ground the rotors would not create backward thrust. You need to lift the front wheels in order to move backwards.

My guess is that if you need to get into and out of a 'hot' strip this approach would minimise the time (and position in the sky) that you would be exposed to enemy fire. Just the sort of flying that might be needed on special ops.


text:
Is multi/quadcopter flight control similar to helicopters?
The wikipedia Quadcopter entry provides some details about its flight control:

Hovering or altitude adjustement.

Pitch or roll  adjustment.

Yaw adjustement.

The following post "Quadcopter flight modes beyond traditional aircraft?" raised to my attention that it had other flight modes apart from those of fixed wing aircraft but I am not so sure with respect to conventional rotorcraft.
Question:
My assumption is that the quadcopter can be flown using conventional helicopter flight control inputs (cyclic, collective, anti-torque, throttle), am I right or is it really another beast with its very own flight control inputs?

I have not the specific knowledge to make a proper answer, but quadrocopter are much more similar to fixed wings rather than helicopters. (among those controls you listed, they have only one throttle per rotor, just for starters)

Given clever avionics and a masochistic pilot, any FBW aircraft could theoretically be flown using a cyclic, collective and throttle. We no longer have to adapt the pilot to crude mechanical linkages but can design the interface to suit the pilot and the mission.

Quadcopters are only ever going to be controlled by computer. They are practical for models and small scale unmanned vehicles, but not full scale aircraft carrying people. See why haven't quadcopters been scaled up yet.

@jan-hudec: I have posted this answer to the question you refered to (scaling up of multi/quadcopter).

@redgrittybrick: I expected the cyclic, collective, anti-torque and throttle to expose the usual facade hiding the complexity under the hood: the interface as you name it. Can you please expand a little and post it as an answer?

As  commented, a quadcopter is generally flown by a computer. Given command inputs, the computer controls the speed of the four rotors, as shown in the images included in the question. The four rotors give the quadcopter a wide range of maneuverability. This is why quadcopters are currently used in many research applications. They can simulate conventional fixed-wing flight, helicopter flight, or something completely different.
A quadcopter could certainly be flown with conventional helicopter controls. The four rotors allow it to be controlled by a cyclic, and the differential rotor torque allows it to be controlled by the anti-torque controls. However, unlike most helicopters, most quadcopters have fixed-pitch rotors. This is probably mainly due to size and cost constraints. The small size of quadcopters, combined with the lift being distributed among four rotors, allows each rotor to be smaller and much more resposive with RPM than a large rotor. This means that instead of collective and throttle, most quadcopters will only have throttle. Controlling the speed if the individual rotors is the end-result of their control algorithms.

Yes a quadcopter can be flown with the four conventional helicopter type controls, which need to be translated into rotor speeds by a control loop, as follows:

Cyclic fwd/aft: fwd rotor spins up, aft rotor spins down equally. Rotor speeds equalise again when the stick angle has been reached.
Cyclic lateral: left rotor spins up, right rotor spins down equally. Rotor speeds equalise again when the stick angle has been reached.
Collective: all rotors spin up.
Anti-torque: clockwise rotors spin up, anti-clock spin down proportionally

Throttle is not required as a separate control, it is effectively the rotor speed which is used in the other four controls.


text:
Why is AVGAS so much more expensive than car gas?
Why is 100LL so much more expensive than gasoline for cars?
I get the fact that it's 100LL, instead of 87, 91, etc.
For example (as of now, 13 Oct 14), the 91 unleaded gas at my local gas station is 3.41USD, and the 100LL at my local airport is 5.49USD (for the self serve).
What's the reason for the huge increase in price between car and aviation gas?

tighter standards, and the tax on leaded gasoline

Is that in USD per gallon?

I'm not trying to be sassy, as I'm sure also weren't, but I think it's fairly obviously I meant usd/gal.

@KeeganMcCarthy For people outside the USA it may not be obvious what units you fuel your aircraft in. For making a comparison it helps to have the units right. So here we go: 2.74 EUR per litre including taxes at the local airport. That is 13.17 USD/gal. You said expensive? :-)

Though it's an old question, let me add prices for car gas. Here, we currently have 1.58€/l = 6.59USD/gal.

The LL of 100LL stands for Low Lead as it has tetra-ethyl lead added as a detonation inhibitor for high-performance engines. Despite the "low" moniker there actually is a lot of it in there, and it's toxic. This fuel is expensive for a number of reasons:

The lead itself is expensive
The lead additive means that a refinery must shut down and clean the lead before producing other fuels, this means more refinery time and therefore more costs
Lead in fuel cannot be allowed to contaminate other fuels so you cannot pipeline leaded fuel. It has to be transported in containers, adding to shipping costs
There are extra controls around the lead in many parts of the world, namely storage requirements and paperwork, all which costs
100LL fuel has many more "aromatic" hydrocarbons than mogas (auto fuel) in order to increase the octane levels and prevent fuel from vaporizing in your lines at high altitude. It's much higher grade, so it costs more
It's a specialist fuel made in much less quantities than other fuels, so a premium is added to ensure a profit

Keep in mind that the Avgas is taxed less than Mogas in most places. It would cost even more if it were taxed the same!
Efforts are underway to come up with a 100LL replacement that will work the same in all engines and situations, which is a hard problem to solve. Whatever fuel they come up with, while having no lead, will still likely be more expensive than mogas just because of the more expensive hydrocarbon mix and the specialist nature of the fuel.

Thanks.  So is the lead just a carry over from when the (auto) engines used to use it, too?

Not at all! The lead prevents high performance engines from tearing themselves apart because of detonation, when fuel explodes in the cylinder prematurely. It's still there because nobody has found a substitute which will work the same that isn't as toxic and expensive.

@GdD Which is exactly why it was used in regular gasoline, right?

Yes, it was added as an anti-knock agent. Modern auto engines are much more sophisticated than most aircraft engines, with automatic mixture control, fuel sensors, etc. Better technology means better detonation control. Aircraft  engines are not only less sophisticated but some also operate at more extreme conditions than cars, like 15,000 feet.

... So cars tooling around downtown El Alto, Bolivia (elevation 13,615ft) need leaded fuels? It actually wouldn't surprise me if they were using leaded fuel, but probably not for the same primary reason airplane engines do.

Auto engines haven't needed lead for decades now, even at high altitudes. I'd be surprised if it was available for cars there.

Not all countries have the same pollution-control standards as the U.S., and TEL is a fairly inexpensive antiknock compound which is attractive to poorer countries. The Chinese are reportedly still making some leaded grades, and it's still used in some other developing countries, mainly Africa.

@SpongeBob: In part yes. The anti-knock compounds used in unleaded fuel also produce pollution that is reduced by the catalyst. Cars were generally upgraded to have catalyst and burn unleaded fuel. Aircraft don't have catalysts, so they still burn leaded fuel (plus there are other slight differences in properties that don't allow using one type of fuel in engine designed for the other).

Modern cars have catalyzers that would be destroyed by lead in the fuel.

You mention that it has more aromatic/VOCs than mogas, but then say that it because of increased stability. Aren't those contradictory? Wouldn't a hydrocarbon liquid that easily vaporizes at ground level/temperature be even more likely to vaporize at higher altitudes? Or does the temperature change negate that?

I didn't say VOCs @zymhan, I said aromatic hydrocarbons. They are called aromatic because many have a sweet smell, not because of volatility. Some aromatics are more volatile than others.

Ah okay, I thought they were synonymous, thanks!

The distillation and refining process for 100LL is more expensive than the process for 91 unleaded.
100LL has a high octane index, and is treated to be less volatile (in particular at high altitudes). 100LL is also perfectly dry in order to prevent icing.
Also, 100LL is produced in smaller quantities than car fuel, so economies of scale are also a factor.
In fact, due to the high price of aviation fuel, certain low-powered engines can be certified to fly with the same gas you put in your car.

there's also far less avgas refined, because there is far less demand than for mogas, and that raises the cost to produce a given quantity (say, a gallon).

Thanks!  The lower demand is something that I didn't think of.  And it's probably the largest reason.

Another factor is you cannot transport leaded gas in the same container trucks as unleaded gas because of contamination (i.e. lead getting into unleaded fuel) issues.

Due to high price of avgas (JetA1 is not expensive) new designs are coming with compression-ignition engines that burn either automobile diesel, JetA/JetA1 or either.

There are non-leaded Avgas variations available. One example is 91/96 UL.   STC-s are available for most or all  normally aspirated engines from Lycoming and Continental. But as the high performance engines on larger machines having turbos needs 100LL it is generally not available.

What's the reason for the huge increase in price between car and aviation gas?

There are many differences, as other answers mentioned, but not as big as the experienced prices show. But by only focusing on the product it's easy to miss cost involved in distribution and sales.
An FBO has at least the same, usually much higher, fixed cost than an average gas station which need to be spread over a much lower number of customers. The fact that a C172 needs more gas (like 50 vs. 15 gallons) isn't much of a relief either.
In addition I don't know of many gas stations providing a lounge, free coffee (and sometimes sandwiches), internet access, flight planing tools and so on, all the way to courtesy cars.
All of this leads to a higher price per gallon than what's caused by the raw gas alone. The effect can be easily seen where auto gas is offered for ultralights, or diesel for capable engines. They carry a visible mark up compared to the gas station next door.
Then of course there's the same effect that lack of competition has on auto gas. Fields with a single FBO are often priced much like the sole gas station on a lengthy stretch of highway ... slightly above average.


text:
What is the "optimal" age of a fighter pilot?
Fighter pilots are always portrayed as young, fit men (and woman), in their 20s and 30s - and most of the F16 pilots I know are that age.
However, today someone, who is over 50 years old, mentioned that he was asked to be an F22 pilot.  His background is astonishing, flying in multiple "wars," and he was even the commander of the Thunderbirds in F-16s for a while.
Even though this man clearly has the background of the stereotypical F22 pilot, would age be a problem?  Wouldn't someone whose body could take more abuse be better suited for that type of assignment?

It depends on what you mean by "optimal".
For a dogfight, the optimal age is probably 30. At this age a person has lots of experience, but still maintains their full mental agility, ability to calculate and energy. In a dogfight your ability to think fast and calculate your next move is the critical factor.
For combat missions, however, 40-45 may be more optimal, because experience, judgement and knowledge becomes more important. Knowing when to attack, how to attack, how to make your approach, how to do your mission setup, and many other things become the product of long experience. For complicated missions in enemy territory, it is better to have someone with long experience, rather than a young hotshot.
Once a person hits about 60-65 years in age, their mental ability to calculate declines significantly. Nevertheless, experience can make up for it in some instances. For example, at age 63 Vassily Smyslov was a candidate for the world chess championship, an amazing accomplishment for a person of that age. Emanuel Lasker took 3rd place at Moscow 1935, a premier chess tournament, at the age of 66, which was practically a miracle. These are exceptions, however. In general, once a person gets into their 60s, their mental ability deteriorates. Therefore, the age 50-55 can be considered the maximum age at which experience and judgement can be used effectively in high speed combat situations.
Complex Mission Role
To investigate my basic assertion that optimal age for a combat mission (not air supremacy) pilot is 40-45 I investigated pilot astronauts. Since astronauts are selected from large numbers of candidates and have short careers typically, it is a safe assumption that their average age is what NASA considers to be ideal. According to NASA Technical Report 1304 the mean age for all selected pilot candidates is 39.90 years old. If we assume a pilot has a 5-year career, then my guess for optimal age matches perfectly with NASA's selection choices. This may be considered strong evidence that for a pilot executing complex missions, the optimal age range is, indeed, 40 to 45 years.
Air Supremacy Role ("dog fighting")
To determine the optimal age for air supremacy ("dog fighting"), if we had access to the USAF's (or other country's) exercise data over time, we might get an idea since we could calculate an ELO rating for each combatant and identify to the top exercise fighters of all time. Unfortunately this kind of data is not released as far as I know, probably because the armed services do like naming or identifying particular soldiers. I do strongly suspect, though, that if such a study was done it would show the "best in the world" air supremacy pilots would be clustered around the age of 30. To check this, I tried summarizing statistics from WW2 German fighter pilots. Using birth data on 470 German aces I generated the following plot:

In this chart, the bar chart is the total number of aces grouped by date of birth. The blue dots are the average number of victories for that birth year group. What this chart seems to show is that performance is remarkably consistent between the ages of 18 and 32, with a slight advantage to the younger pilots, but then declines significantly. The 1922 group was significantly skewed by Erich Hartmann. If we eliminate him as an outlier, the top group is the class of 1920 which would have been between the ages of 19 and 25 during the war. This seems to show my original guess was wrong, and that dogfighting is dependent more on quick reflexes and fast thinking than on experience, which hands the edge to the young.

It might also be worth mentioning, that while strength decreases with age, endurance increases. And since strategic mission can easily be 12 or more hours, this also favours older pilots (it favours them more in bomber pilot positions; B-2 missions may easily take 40 hours though that has two pilots and enough space to lay down and take a nap).

Auto racing, as an analogy, is similar. Generally, shorter races upwards of 2 hours are dominated by younger drivers, while the endurance races of 12- to 24-hours are dominated by older drivers in their 40s and 50s.

It's interesting that you claim 40-45 is the optimal range and 50-55 is the maximum, considering that what Pondlife posted shows a younger range for fighter pilots, and that airline pilots don't have to retire until 68 in Japan now. Astronauts are a bit specialized to be using as a comparison.

1 ... Finally added a source and expanded your answer. Can you source your assertions of the third paragraph? (Note I have found several sources confirming you, but I hesitate to edit your answer.)

"The 1922 group was significantly skewed by Erich Hartmann." -- I had to chuckle at that. ;-)

@TylerDurden - while making an attempt to quantitatively assess the impact of age on fighter pilot performance is commendable, I think the conclusions you are drawing are somewhat spurious. First, only studying fighter aces offers little glimpse into the generalized performance of fighter pilots. That is to say, only a small percentage of combat pilots score kills and, of those, only a small percentage become aces; in other words, your sample is unlikely to be representative of the fighter pilot population as a whole.

Secondly, only looking at kill totals and averages is unlikely to allow you to draw the correct conclusions. Kill totals are inadequate because they work heavily in favor of those pilots who had more opportunities to engage the enemy. Straight averages alone are also insufficient in comparing outcomes between samples (even assuming that samples are comparable in all respects other than your chosen grouping criteria) because they ignore outcome distribution – see the Hartmann example.

Ignoring all other considerations, should you wish to compare differences in fight pilot productivity at different ages, you would, at a minimum, need to perform a one-way ANOVA (post-hoc testing too) of kill frequency grouped by actual pilot age at time of record.

Third, in only looking at pilots’ birth years, many other contributing factors are swept under rug. In order to properly assess fighter pilot performance you would likely need to resort to considerably more complex modeling techniques and also include factors such as pilot combat experience (Vietnam-era USAF studies show that a pilot’s effectiveness and chances of survival increase dramatically once they survive their first 10 combat missions), opposition strength, wealth of scoring opportunities, quality of training, equipment, support infrastructure, leadership and tactics, etc.

Please do not view my comments as an indictment of your efforts; you’ve obviously put quite a bit of time and thought into them, but the conclusions overlook some very important considerations. I also apologize for the run-on comments.

@habu, in terms of answering the question, he did provide very good support for his answer, and although you make a valid point, if he did include those other points such as "pilot combat experience (Vietnam-era USAF studies show that a pilot’s effectiveness and chances of survival increase dramatically once they survive their first 10 combat missions), opposition strength, wealth of scoring opportunities, quality of training, equipment, support infrastructure, leadership and tactics, etc." then he would have deviated too far off topic.

@Jihyun with all due respect to the poster's efforts in gathering, analyzing, and charting the data, the picture he's presenting is very incomplete and leaves out several confounding factors that can have material impacts on the metric used to measure pilot proficiency. In fairness to the poster, gathering such data would require the kind of effort expected of a PhD dissertation, but relying on a simple univariate analysis of kill totals to answer this question risks either falling prey to spurious correlations or lacking sufficient power to dismiss alternate hypotheses to explain the outcomes

Consider the following rebuttal to the conclusion that, based on kill totals, "dogfighting is dependent more on quick reflexes and fast thinking than on experience, which hands the edge to the young". The 19-25-year old cohort (born between 1913 and 1923) was in the prime of their fighting age between 1939 and 1945 and would, therefore, have seen the most action, giving them more opportunities to score than their older peers who would have moved to command, admin, training, or otherwise been removed from flying, as well as their younger peers who joined the fight too late to see as much action

True. I too would have preferred a more open perspective, possibly including examples of training and improvements of, not to mention a certain non-comparability between the slower propeller based fighting tactics of WW2 and more energy and technology based combat of today.

I can't directly tell you what's "optimal" because I don't know what that means, but the US Air Force does provide some basic demographic information so we can indirectly see what they consider to be a good age mix for fighter pilots. I used their report tool to get data on fighter pilots1 and it shows this breakdown:
17-24 :    43
25-34 : 1,341
35-44 :   968
45+   :    83
Total : 2,435

This isn't very fine-grained, but on the other hand we can immediately see that your example of someone over 50 is really at the extreme end of the curve: only 3.4% of fighter pilots are over 45.
Of course, this only applies to the USAF and it doesn't tell us anything about other branches of the military or about what other countries do. But we can say that the USAF appears to consider 25-34 to be the 'optimal' age: 55% of their fighter pilots are in that bracket.
(I should add a small disclaimer that I know nothing about the military, so hopefully I didn't make any stupid mistakes in pulling or interpreting the data.)

1 To be specific, the search criteria I used were: Active Duty Air Force only (excludes Air National Guard, Air Force Reserve and civilians); officers only; AFSC 11FX specialty (fighter pilot) only. The data is from April 2015.

This says nothing about the optimal age. It only defines the current age distribution in the US AF, which is not necessarily optimal. Military promotions and assignments are based on many factors, including family considerations, that have nothing to do with whether the personnel is of the "optimal" age.

One issue - At some point, you get promoted or are kicked out of the air force. As you reach higher ranks, there are fewer slots for you in a flighter squadron, because you won't see generals flying anything normally.

Also, different roles or tasks have potentially different optimal ages, as I point out in my answer, so giving blanket statistics tells nothing about the role breakdown.

@TylerDurden As I said, I know nothing about the military. But, these numbers are for the 11FX (fighter pilot) specialty only, and I find it hard to imagine that the USAF would designate someone as a fighter pilot for purely organizational or seniority reasons. Even if they do, the numbers show that the USAF has created an organization where most fighter pilots are 25-34; whatever their reasons are, the USAF is presumably smart enough to create an organization that fits their needs, which strongly suggests that for the USAF at least, 25-34 is the preferred age.

By age 45, pilots are becoming eligible to retire, and plenty do.  That doesn't mean that they couldn't be amazing fighter pilots for many more years, simply that they have other options available.  Airline pay + retired pay >> AF pay at that point.

Does the data on pilots with that AFSC include the ANG and AFRES?  Pilots there may exhibit different demographics -- less of an up-or-out dynamic, and the ability to fly part-time in the Guard/Reserve + also work as an airline pilot.

@RalphJ I took the data for "Active Duty Air Force", officers only, so ANG, AFRES and civilians are not included. Since my knowledge of the USAF is about zero, you or anyone else who knows more than I do might like to play around with the reporting tool, you might get something more detailed or interesting out of it than I could.

Also consider that for some (perhaps many) people, there are many negative aspects to military life.  One might put up with them for a while because of the benefits (like getting to fly fighter jets), or a sense of duty, but eventually a good many people might want out, regardless of their actual capacity.

In addition of Pondlife's excellent answer, there are a few points I would like to mention.

There is no specific age at which a person no longer remains a fighter pilot. It relates more to that individual's physical and mental capabilities.
In air force/military, when a person reaches senior ranks, there are other responsibilities which are added.
When a person is no longer a fighter pilot, it does not mean that they cannot fly an F-16, F-22, or any of the airplanes they used to fly. It just means that they are not combat ready anymore. They can still teach to fly or fly at air force demonstrations etc. In fact, the air chief marshal of a country (long retired from combat flying) leads the air force squadron on their annual ceremony.

From the numbers Pondlife mentioned, it does appear that the optimal age of about 95% of pilots is in fact in 25-44 range, but many are able to keep the abilities beyond that age range.

Interesting views and topic. I landed here by chance. I'm 46 now and a couple years ago I flew an F-22 into combat (I was older than 41 but younger than 45). They needed someone with special experience due to the delicacy of the operation. I say as long as you are healthy and pass the physicals etc, I can see a 53 year old combat pilot but by 55 you should hang up your combat hat. I consider myself an experienced combat pilot. Yeah I use combat vs. fighter. At 60 you should be flying your personally owned aircraft.